Patent document CH-A-457 291 has already described a sealed pushpiece for a diver's watch comprising an axially movable part, slidingly mounted on a tube fixed to the body of the case, partially enveloping such tube and exhibiting a central core penetrating into the tube, at least one sealing element between the tube and the core and a return spring pushing the movable part back towards the exterior.
The same document indicates that in most sealed pushpieces mounted on diver's watches, it is necessary to use a return spring sufficiently powerful so that the pushpiece is not compressed by the heavy pressures prevailing at great diving depths. Such solution evidently has the disadvantage of requiring an excessively strong manual pressure on the pushpiece when the latter is operated out of the water or at small depth.
To overcome this difficulty, the pushpiece of the cited document proposes means permitting use of weaker springs for like depths or permitting attainment of greater depths with the present springs. The means indicated to arrive at such result consist in having the annular space comprised between the core and the enveloping part of the movable part communicate with the exterior of the pushpiece by a space left between the enveloping part and the tube and at the same time by at least one opening formed in the upper part of the enveloping part. Thus, during the dive, water penetrates easily into such annular space and the pressure of the water at the exterior of such space increases practically at the same time as the pressure of the water at the exterior of the pushpiece, and so relieves the return spring.
The means recommended hereinabove are however still insufficient to reduce the return force of the spring to a suitable value if the pushpiece must resist a very high hydrostatic pressure, for example that prevailing at a depth of 500 meters, which is to say according to ISO standard 6425, 50 bar (1 bar=10.sup.5 Pa=10.sup.5 N/m.sup.2 =0.1 N/mm.sup.2). This results from the fact that the cross-section of the central core of the movable part on which the hydrostatic pressure acts is relatively large, the section to be taken into account being that which is surrounded by the O ring seal. One can calculate the force F to which the spring must resist at pressure P if r is the radius of the core: EQU F(N)=.pi..multidot.r.sup.2 (mm.sup.2).multidot.P(N/mm.sup.2).multidot.s (1)
in which s is a safety factor greater than 1, for example 1.25.
In known pushpieces, such as that of the cited document, the diameter of the core is on the order of 2 mm (from whence r=1 mm). If such pushpiece must resist a pressure of 50 bar, the force of its return spring must be: EQU F(N)=.pi..multidot.1.sup.2 .multidot.5.multidot.1.25=19.6 N
which is the force which it will be necessary to exert on the pushpiece when the timepiece is out of the water. This force is considerable and difficult to accept. From whence the idea of the present invention to diminish the diameter of the core, thus to diminish the average diameter of the seal which surrounds it. This entails as corollary proposal a return spring the average diameter of which is greater than the average diameter of the seal since without such, the spring would not respect the design rule which requires that the average diameter of the spring be equal to or greater than 6 times the diameter of the wire. As here it concerns a diver's watch, one assumes as necessary fact that the return spring be located in a sealed zone of the pushpiece in order to avoid corrosion of such spring.